GroovySWT is a wrapper around SWT, the eclipse Standard Widget Toolkit. It allows you to easily write Eclipse SWT applications by using Groovy's builder mechanism.
Here is some SWT code using native Groovy:

When you run this (see below for setup details), the result looks like:

Here is the same example using SwtBuilder - note that this example does more because it actually prints some text 'Hello' to standard output when you press the button:

Here is another example which explicitly creats a script class and assumes guiBuilder is passed in via a binding:

Further information

The sources (and README.txt telling you how to set up the libraries and dll's) can be found here: